The invention relates to an electro-optical time display device, of the type in which the time information is represented in analog form by the simulation of hands rotating over an hour dial.
A display device of this type is described in published British patent application No. 2,014,337. Simulation of the hands is effected by electro-optical means which comprise, over the entire surface of the dial, an electro-optical material, comprising for example liquid crystals, enclosed between electrodes which are distributed in two layers and which are individually placed under the control of time counting means. At positions where the electrodes of the two layers are simultaneously excited at opposite potentials with respect to a neutral potential, so as to subject the electro-optical material to a sufficient electrical field, that material appears in a contrasting mode against the remaining background of the dial. In order to simulate movement of the hands, the arrangement is thus controlled to provide sequential excitation of electrodes in the form of sectors or dial segments which are distributed all around the dial, at the speed of forward movement of the hands as determined by the counting means.
In order to provide a representation which is comparable to that of the hands of conventional clocks and watches, in particular the minute and hour hands, it has been necessary to form the electrodes of the layer on one side of the electro-optical material in the form of radial segments which are distributed all around the dial, and to form the electrodes of the other layer, on the other side of the electro-optical material, in the form of sectorial plates which are distributed in two concentric rings. Each ring thus includes a plurality of plates which are distributed all around the dial, but each plate covers a sector of an angular extent which is a multiple of that of each segment. The minute hand is simulated by a radial segment which is made visible over its entire length by simultaneous excitation of that segment and both of the plates which are opposed to it, namely one plate of the internal ring and one plate of the external ring. The hour hand is simulated by one segment or by two adjacent segments which are excited simultaneously, but only over a part of their length, with simultaneous excitation of the opposed plate which belongs to the internal ring, and not that in the external ring.
It will be appreciated that the sequential control of the electrical supply connections can become highly complex, depending on the number of separate electrodes, although that number must be sufficient to permit reasonable representation of the position of the hands and the movement thereof. There are preferably 60 segments, so that controlling the minute hand involves causing the simultation and therefore excitation to pass from one segment to the following segment every minute. In other words, the displayed minute hand changes position every minute. In order to ensure that the number of electrical connections is acceptable in the construction of clocks and even watches, the above-identified British patent application provides for a supply in series by the same electrical circuit for segments associated with different opposed plates, this being effected in correlation with a multiplexing control system.
There are ten groups of radial segments, each group comprising six successive segments opposed to the same pair of sectorial plates. Furthermore, the dial comprises ten of these pairs in the same manner as it comprises ten groups of segments. The segments are electrically connected in series in tens, one of each group, to form meandering circuits, that is to say, the circuits meander between segment connections alternately at the radially inner and radially outer ends of the segments. At the boundary between two sectors, the two adjacent segments which are respectively associated with different plates belong to the same meandering circuit (and may therefore be joined as a double width segment).
The above-identified British application also describes, with all the details required, the manner in which such a time display dial may be controlled by multiplexing logic circuits on the basis of data relating to the state of time counters, in binary form. The display of a suitable segment in the correct sector, on the internal ring alone if the display involves the hour hand, or on both the internal ring and the external ring if the display involves the minute hand, involves exciting the proper meandering circuit and simultaneously exciting the proper sector-shaped plate or plates on the other face of the dial. The published British application discloses how to obtain mean electrical fields between those electrodes which determine whether there is or is not a display, it being deemed sufficient to have four square wave forms for the plates, which are phase-shifted through 90.degree. relative to each other, and, for the meandering circuits, four other wave forms which are derived from the same wave form having three plateaux, also by phase shifting through 90.degree.. The combinations of these wave forms having two and three plateaux provide all the control actions required for simulation of the different positions of the hands of the dial.
However, the watches which it has been possible to contruct in accordance with the foregoing are still seriously deficient in regard to the quality of simulation. In particular, the position of the hour hand, when the time is right on an hour, is spaced at such a distance from its normal position, relative to the corresponding figure, that reading of the time becomes uncertain. In addition, a representation which is similar to that given by conventional dials with hands requires that the hour hand must be formed by two segments which are disposed side-by-side in the internal ring in order clearly to distinguish it from the minute hand and, in this case, it has not been possible hitherto to design control circuits which retain all the desired degree of simplicity.